Any Hints on Improving CW Reception?

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  • Updated 7 months ago
I'm finding it very difficult to copy weak CW signals, both on the Maestro and with SSDR running on the PC. There is a great deal of noise, and tightening up the bandwidth makes it ring quite a bit. I have the RF gain all the way down to reduce IMD (which probably doesn't apply to an SDR). Strong signals are no problem, but weak signals are very difficult to hear. My FTdx5000 seemed to do a better job, which is surprising.

Is there anything I can do to bring the signal out of the noise better?
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Frank Kirschner

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Posted 7 months ago

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Andreas Junge, Elmer

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Frank,

for me it works well with the APF at mid setting and then take the AGC-T down until the noise starts going down a tiny bit. That's when the CW signals start popping up. 

In my setup the AGC-T is between 35 and 50. Your's may be different. 

73, Andreas, N6NU
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George Molnar, KF2T, Elmer

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Sounds like you're doing some good things. Try the APF (audio peaking filter), in conjunction with a narrow CW filter. You can set the APF characteristics in the radio setup menu. Helps reduce ringing.

 

AGC-T is something to look at closely. There's usually a sweet spot that reduces noise while letting the signal through beautifully.

 

I usually use Fast AGC, but some folks prefer medium. Depends on the code speed. 

 

On HF, you probably don't need preamps on, so make sure they're off and not contributing to more noise. 

 

Hope that helps!

 



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Ernest - W4EG

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Geo,
You took the words out of my mouth. 
That is exactly what I have to do sometimes. I try adjusting the controls you mentions; if I am interested in the conversation or if I am trying to work something exotic that I have not worked or rarely hear.
73
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Michael Walker, Employee

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Hi Frank

SDR's do not have an RF Gain control.  :)  I suspect that you are using the AGC-T control like an RF gain control and that is the wrong way to use it.  

This should help you out.  

We discuss AGC-T in the manual on Section 13.1

Mike



13.1 AGC THRESHOLD

Automatic Gain Control (AGC) is a feature which automatically adjusts the Slice Receiver’s audio gain (volume) based on the strength of signals in the receiver’s passband filter. The goal of AGC is to amplify weak signals and attenuate strong signals so that they all lie within a comfortable listening range.

The receiver Automatic Gain Control Threshold (AGC-T) can be adjusted for optimum performance in noisy or quiet environments. The AGC-T sets the maximum gain applied under any circumstances. Since the noise floor is relatively constant on a given band at a given time, the AGC can be adjusted using the threshold control so that the AGC never applies gain to noise, but it will apply gain to signals just out of the noise. In doing so, the AGC can reduce the level of noise you hear, and help signals pop out of the noise.

The AGC system in SmartSDR is a dual track system, meaning that it can track both slow and fast increases in signal strength, making appropriate gain correction decisions in the presence of each. The speed of the AGC (FAST, MED, SLOW) determines how quickly or slowly the AGC recovers after attenuating a strong signal. You can easily hear this by tuning to a CW signal and going through the three settings. On FAST with a strong signal you can hear the gain pump up and down while on SLOW it recovers after a longer pause once the signal stops. When the signal stops, you will hear the noise floor increase as the gain returns.
How to Set AGC for Different Operating Conditions

If you are listening to a loud voice signal, AGC SLOW will resist increasing the gain between syllables and therefore reject most of the noise which is at a level far below the signal. FAST and MED provide faster levels of recovery for situations when you want the system to more closely follow the dominant signal in the passband. Any time you have a very strong signal that causes the gain in the AGC to be reduced, you could experience a loss of gain to a weak signal you are listening to. The filter passband edges, which are continuously adjustable, and TNFs can be used to eliminate signals that might interfere with AGC operation.

The operator might prefer to use SLOW settings when rag chewing in a high signal to noise environment where there isn't much QRN and the noise floor is stable. This keeps the gain at more of a constant level that is less distracting. If the operator is trying to pull a weak CW signal out of the noise, they may prefer to use FAST mode to quickly ensure that the long-term average of the noise floor doesn't overcome the signal and prevent it from being heard. MEDium is a reasonable compromise.


To adjust the AGC Threshold, tune to a quiet spot between stations. Starting with the AGC-T at a high value adjust the AGC-T slider to the left (lower gain values) until the background noise just begins to decrease. This is the AGC-T "sweet spot" or the "knee" of the AGC algorithm. Depending on band conditions, if the AGC-T is set below 50, you may have to compensate for the loss in audio gain (volume) by increasing the Slice or master AF volume to a higher value. When you get the knee and AF volume adjustment correct for the band conditions, it will keep the volume of strong signals constant which will allow weaker signals to be heard even with AGC in FAST mode. Thus AGC-T is one of the most important adjustments, and often overlooked, to achieve the maximum weak signal receive performance out of the FLEX- 6000 series SDRs.
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Kevin

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Hi Mike... The Flex SDR does have RF Gain. Is this actually doing something different than the RF Gain in a non-SDR radio?



Kev
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Frank Kirschner

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What does the RF Gain control do if not control the RF gain?

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Al / NN4ZZ

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The RF slider does control the RF gain or attenuation of the preamp.  Since the preamp is not generally needed on HF (below 10 M) it is usually off and easy to forget about this setting.  Or that it even exists.  

Here is a good post about when to use it:

https://community.flexradio.com/flexradio/topics/when_do_i_need_to_use_the_flex_6000_rf_preamp

Regards,  Al / NN4ZZ  
al (at) nn4zz (dot) com



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John - WA7UAR

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That link didn’t work for me Al — was this what you were wanting to refer to?
https://helpdesk.flexradio.com/hc/en-...
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Al / NN4ZZ

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Hi John,
Not sure why the link didn't work for you but pasted in below is the content of the post by Gerald.  It is similar to the helpdesk entry.

Al / NN4ZZ  
al (at) nn4zz (dot) com
6700 & SSDR-W  V 2.4.9
Win10


https://community.flexradio.com/flexradio/topics/when_do_i_need_to_use_the_flex_6000_rf_preamp

by Gerald Youngblood, K5SDR 

Answer: On HF - almost never. I had the opportunity in April to take a pre-production FLEX-6700 to Rob Sherwood's (NC0B) low noise remote shack East of Ault, Colorado. [Ed: For those who don't know who Rob Sherwood is, please check out his extensive research and unbiased reports on amateur radio receiver performance at: http://www.sherweng.com/table.html ] Rob had been telling me that his site matches the Quiet Rural measurements shown in the ITU-R P.373.7 Radio Noise charts for HF. He also has an amazing antenna farm with single band antennas for each band mounted on 70 foot towers, as well as, a Marconi 160m antenna. Rob had also told me that many radios have problems with inter-modulation on his 160m antenna due to large medium wave broadcast stations in the area. 

My first goal was to understand 'if' and 'when' RF preamplification is needed on HF with the FLEX-6000 series. The second goal was to understand if the two -7 dBm (S9 +66 dB each) AM BCB stations would cause any adverse affect to the radio on 160m. 

We arrived at his QTH around 10PM and quickly got the radio up and running on his network after Rob remembered to connect his fiber to copper network bridge. Fortunately, there were openings over the North Pole into Siberia and Russia on 15m and 10m late into the evening. These were weak signals at his location so I am sure you would not be able to hear them at any urban location. The way Rob determines whether a preamp is needed is to connect and disconnect the antenna. If the noise floor rises by 8 dB or more with the antenna connected, then you don't need or want a preamp. The preamp would actually reduce dynamic range on any radio. 

What we found was that no preamp was needed on any HF band until we got to 10m. On 10m, 10 dB of preamp gain was almost enough and 20 dB only improved SNR slightly. Based on testing at Rob's lab in Denver, I learned that the preamp noise figure was actually 4 dB higher than its specified performance. Upon my return, I found and easily fixed the problem before we went into full production. That probably means that 10 dB gain would have been sufficient on 10m. 

An important phenomenon that most people don't realize is that even in the quietest locations on earth, noise propagates with the band opening. In other words, noise is RF and RF propagates just like intentional signals. We saw this easily on 15m where pointing the antenna away from the opening caused the antenna noise to drop below the radio's internal noise floor. When we pointed the beam toward the opening, the noise floor rose more than 8 dB above the receiver noise showing that a preamp was not needed on 15m. 

The next thing we did was to connect the 160m Marconi. I placed one receiver slice in the AM broadcast band to intentionally disable the 1.8MHz high pass filter. That means that the radio was wide open from DC to 70+ MHz on a very efficient antenna. The bottom line was no overload or inter-modulation from the two -7 dBm stations. Since no preamps were needed, that means that one could simultaneously receive AM broadcast and all but the very weakest signals on 10m without a preamp. In fact, the radio is capable of receiving up to +9 dBm (S9 +84 dB) signals without overload.



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Terry K8EET

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You can try playing with the RX equalizer. Lower everything but the 500 or 1000 Hhz settings depending on your preference. 
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Jeff, W4DD

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I found the same to be true on my 6300 on 10, 15, and 20m vs my Orion II. I had to turn the 6300 preamp on to make them equal. The Sherwood performance web page seems to indicate a preamp may be needed for weak signals and I found a previous thread where it was suggested.
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Sherman Banks

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I also had trouble copying CW signals. SSB was no problem. At first I thought I was going deaf but I had no problems with the K3. Turns out the AGC-T setting is the most critical setting. Follow the manual's instructions. Also try adjusting the AGC speed. Turn off NB, NR and WNB. APF can help.

There is a request to automate the AGC-T setting since it is critical. Thank goodness the Maestro has a knob for it to make it easy to change on the fly. See the link below.
https://community.flexradio.com/flexradio/topics/option-to-automate-the-agc-t-setting
 


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Duane, AC5AA

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I set the first button on my FlexControl for AGC-T which makes it really easy to adjust and then jump back to tuning function.
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Clay N9IO

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Great idea Duane.
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dan flynn

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Frank,
         Don't forget mechanical resonate speakers.   Here's a link to building one. (there are plenty other designs if you google around..) 

https://youtu.be/MF8bk6958Hg

Also,  here's a starting point on the science.

https://people.seas.harvard.edu/~jones/cscie129/nu_lectures/lecture3%20/ho_helmholtz/ho_helmholtz.ht...


Dan ww3n

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AC9S

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You might try what I do which is go to the Phone/CW menu (on the Maestro) and adjust the CW filter latency slider to minimum.  This widens the filter skirts substantially, but that is usually not an issue with weak signal reception.

My standard approach to 160 now is to set the filter latency to minimum and use a 50hs filter width.  In addition, of course, I use the APF and fiddle with the AGC-T.

In the last 160 meter CW contest there were lots of very weak whisper traces that were totally unreadable with my standard 250hz automatic latency.  50hz and minimum latency allowed me to work a dozen or so previously unreadable Europeans.  50hz and APC pop the signals out of the noise and minimum latency stops the ringing.  You will hear stuff off the sides with the wide filter slope.

Keith - AC9S
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Jamie WW3S

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Depends on the band, and antenna....someone mentioned you don’t need a preamp on the hf bands....I use it on 160 with my rx loop and have worked a bunch of dx I couldn’t hear on my icon or k3....crank the agct down to next to nothing, turn the af gain all the way up.....ymmv....
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bahillen

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Frank
You have to spend some time with weak signals under different situations. For me NR has been significant. As suggested the EQ can make a big improvement in attenuating high and low frequency noise.

What works for 80m May be different than 15 m. Your type of noise make different things work more or less. The mode you are operating make a difference also.

Since the he noise blanket subject was raise here recently, I have been trying different options, settings and gain. This lead me to realize that flex has many tool to abate noise. Learning how to use them takes an invest of your time and skill. I have pulled stations out of the noise that were well below the noise. That said there is always room for improving software and inventions. New software releases can change the fundamental behaviors with the same hardware. If your radio works with S9+10db noise I would keep it. If your Flex doesn’t deal with your noise like your last radio, document it and submit a help desk ticket, maybe it will help all of us on the long run.

We often want more in the radios we acquire. The downside of that is the options that the Flex has requires that we learn how to choose and operate what we want. Engineers in the past decided for you what settings to make and thus restricted the capability to suit tha majority. My FT1000MP had limited capability to pull someone out of the noise, you could hear him or not.

This is not only a radio but a communications system and does some unbelievable things. But there is a comfort level with what he have used and is maybe not for everyone. My National NCX-5 from 1961 still works, very simple to operate and can be fun to operate.

I would like to hear back from you Frank what you find as you implement the suggestions provided above and how the FTdx5000 compares. What kind of noise is in your environment. If you add what you find, six months from now someone will read this thread and learn from your experience. I personally am interested as others would be too. When you write down on a pad what you find, you will be surprised additional things you learn or a couple other aspects you want to look at.

73
Bill W9JJB
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Frank Kirschner

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Thanks for all the suggestions. The AGT-T, which I had mostly been ignoring, helps considerably, as does using the APF instead of cranking down the bandwidth of the slice.

I also found that reversing the connections on one of the speakers helps. With identical signals going to both left and right channels, the desired signal was cancelling right at the center, where I sit. Reversing one causes the signal to add in the center.
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Stan - VA7NF

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Well, for contesting with really in the mud CW I find:
  • Don't add anything with resonance, speakers, peaking filter etc - It just muddies the CW.  Especially if it is cross polar as that stuff has so much aurora distortion already.
  • Narrow the filter to below 200.  50-90 is my preference - That reduces the noise significantly BUT the AGC will bring up the background and because of the narrow bandwidth, any noise will sound like signal and muddy the signal again.
  • Everybody talks about the AGC-T, that is the remaining best tool EXCEPT bring it down slightly more than normal, that will reduce/eliminate the AGC action on the background noise so the CW pops out that little bit more.